The simplicity of stars is sometimes called
"the Russell-Vogt Theorem." For the stars near the sun, even the initial
composition is usually very similar, so their properties depend on just mass and age! The
evolution of stars is then just a question of determining how ones of a given mass change
their properties as they age.

Very young clusters of stars have all or most
of their members above the main sequence, since they are still contracting and have
not reached stability with all power derived from thermonuclear fusion. Eventually they
settle on the main sequence.

Why is there a main sequence?

If there were no thermonuclear fusion of
hydrogen into helium, a star would continue to shrink under gravitational
attraction as it released energy. There would be no stable period in the
life of the star, and if we looked at stars over a range of ages and masses
we would expect them to be scattered all over the H-R Diagram. Thermonuclear
fusion heats the inside of the star, creating pressure that stops the
collapse and producing a long period of great stability under hydrostatic
equilibrium that defines the main sequence. (From B. Kellet,
http://ast.star.rl.ac.uk/hr.html)

The most massive stars, 60-80 times as massive as the sun, lie on the
main sequence at the very high luminosity and high temperature tip. The
great outpouring of photons from stars more massive than about 100 times the sun tears
them apart, so they never manage to become stable.

Stars that have exhausted their core
supply of hydrogen swell up and become very luminous. They are called red supergiants and
giants, and their large size, relatively cool temperatures, and high luminosities mean
they are plotted on the HR diagram above and to the right of the main sequence.
Fundamentally, they do not lie on the main sequence because they are no longer powered by
core burning of hydrogen.

-- After the star becomes a red giant, its core continues to contract and
heat up -- contraction continues so long as insufficient energy is generated to balance
the force of gravity.

-- Eventually the core temperature will
reach ~200,000,000oK, hot enough to trigger another set of nuclear reactions,
called the triple-alpha process (an alpha-particle is a He nucleus):

4He + 4He ---> 8Be +
photon

8Be + 4He ---> 12C +
photon

That is, these stars start to burn helium as well as
hydrogen and build up the combustion product carbon in their cores. Here is a sketch of the interior structure of the star at this stage.
(From Chaisson & McMillan, Astronomy Today)

These reactions produce a large burst of energy when they start -- the
He in a star's core may "burn" in just a few minutes (or even seconds). This is
called the helium flash. The energy released takes thousands of years to diffuse
outwards and leave the star.

If the star has enough mass, the core will
contract, heat up, and trigger further reactions after the helium in the core has all been
converted to carbon. A star must have a mass greater than 8 M for the following reactions to take place

12C + 4He ---> 16O

12C + 16O ---> 28Si

16O + 16O ---> 28Si + 4He

28Si + 28Si ---> 56Fe

This sketch shows the structure of the star when it is accumulating
an iron core, as in the last of the reactions listed above. (From Chaisson
& McMillan, Astronomy Today)

The star resembles an onion -- the core might be converting Si to Fe
with each step outward yielding a layer burning another set of elements.

Because Fe (iron) is the most stable atomic nucleus,
reactions cease when 56Fe is formed.

By compiling data on many pairs of stars,
we know that the luminosity of a star on the main sequence increases
rapidly with mass:larger masses
imply higher luminosities because higher mass stars have higher
central temperatures and hence convert H to He faster.

A star's lifetime on the main sequence is also related to its mass

High mass stars burn H rapidly.

Therefore, they have short main sequence lifetimes.

Low mass stars burn H slowly.

Therefore, they have long main sequence lifetimes

Higher mass stars have higher surface temperatures that are a
reflection of their higher internal temperatures -- the more massive the star, the larger
the gravitational pressure at the center and the higher the central temperature and
pressure. Nuclear reactions proceed more rapidly if the temperature is higher so massive
stars burn their nuclear fuel more quickly and evolve more quickly.